The present invention relates to a continuous steel girder bridge applying a temperature gradient to improve the bridge behavior due to the (xe2x88x92)bending moment acting near the inner supports of continuous steel girder bridges. More particularly, it relates to the continuous steel girder bridge constructed by temporarily applying an artificial temperature gradient to the steel girder of the composite girder bridge from shortly before the slab concrete is casted and until full composition effect between the girder and slab takes place. By removing the artificial temperature gradient after composition is completed, prestressing effects like the followings are acquired; cracks can be prevented by offsetting the tensile stress acting on concrete slab resulted from the (xe2x88x92) moment, reducing the amount of the slab reinforcement bar and decreasing the cross-section of steel girder are also possible.
Hereafter, the composite bridge of prior art will be schematically described, referring to FIGS. 1A, 1B, 2A and 2B.
Referring to FIG. 1, the composite steel bridge of prior art is comprised of reinforced concrete slab 1, a steel girder 2 supporting the slab 1 and shear connectors 3 disposed over all length of the steel girder 2 to combine the steel girder 2 with the slab 1. Above components operate as a composite section. In this case, stresses of steel girder and concrete slab under design loadings have to be restricted under the allowable stresses. There is no particular problem in the case of a simply supported beam, but in the case of a multispan beam, the (xe2x88x92) bending moments develop near the inner supports due to the live load as well as dead load and cause tensile stresses in the concrete slab and upper section of the steel girder. To prevent the tensile stresses from developing in the concrete slab, it is necessary to arrange the longitudinal reinforcing bars or to adopt up and down construction method or prestressing method using prestressing tendon. All of these traditional techniques make construction process complicated.
FIGS. 2A and 2B show the typical bending moment diagrams of general 2-span continuous bridge and 3-span continuous bridge, respectively. The tensile stress occurs in the concrete slab under the (xe2x88x92) bending moment, causing tensile cracks. In the design and construction of composite bridges, additional reinforcements in the longitudinal direction are placed to resist the tensile stresses acting on concrete slab. However, the tensile cracks of the concrete slab can not be prevented. Sometimes in designing this kind of continuous composite bridge, sections under the (+) bending moment are assumed to be composed and sections under the (xe2x88x92) bending moment are assumed to be non-composed. However, the tensile stresses still develop due to partial composite action between the slab and girders.
According to the previous studies, by placing reinforcement bars in the longitudinal direction by more than 2% of the concrete slab section area, and by keeping the ratio of the total circumference of the reinforcement bar to the cross-section of the concrete slab above 0.045 cm/cm2, the tensile crack of the concrete slab can be limited to allowable crack width. Also the longitudinal reinforcement bars have to be extended up to the compressive stress region.
Therefore, in practice, the longitudinal reinforcing bars are excessively arranged in the (xe2x88x92) moment region, thereby increasing the amount of steel needed and decreasing the workability at site.
It is, therefore, an object of the present invention to provide continuous Composite steel girder bridge using temporary temperature gradient to generate compressive stress in the concrete slab near the inner supports, offsetting the tensile stress due to the (xe2x88x92) bending moment caused by the live and dead load, and thereby preventing the tensile cracks.
Further, another object of the present invention is provide continuous Composite steel girder bridge and method for constructing the same capable of decreasing the amount of longitudinal reinforcing bars needed to limit the tensile crack size, and reducing the cross section of the steel girder by applying a temporary temperature gradient to the steel girder before the composite effect takes place.
Furthermore, the other object of the present invention is to provide a method of constructing continuous composite bridge applying an artificial temperature gradient with simple equipments.
In order to accomplish the objects mentioned above, the continuous composite girder bridge constructing method using temperature gradient comprises: a first step of placing a continuous steel girders over predetermined spans; a second step of providing heating source over the upper and lower parts of said steel girder to generate a desired temperature gradient; a third step of composing after casting and curing the concrete slab on the continuous steel girder having the desired temperature gradient; and a fourth step of offsetting the tensile stress in the concrete slab occurring due to the dead load and live load, through the compressive stress simultaneously occurring in the upper section of the steel girder and the concrete slab, by removing the temperature gradient after said third step is completed.
Further, the apparatus which generates the temperature gradient is placed on the steel girder. It is comprised of the heat source, thermo-sensor that output the temperature of the heat source and a system controller to keep the artificial temperature gradient steady, based on the information collected from the thermo-sensor, until composition effect takes place.
Furthermore, the present invention presents a continuous composite bridge constructed using temporary temperature gradient yielding the compressive stresses in the concrete slab which offsets the tensile stress caused by the dead and live load.